We first discuss high-temperature transport.
The inset of Fig. 4(a) shows the temperature dependence of
the anisotropy of thermopower \(Q_b/Q_a\) .
The anisotropy \(Q_b/Q_a\)  decreases with
increasing temperature and becomes close to unity near 600 K.
This behavior is well consistent with the localized model,
because the thermopower at high temperatures can be expressed by using
chemical potential \(\mu \)  as

Q=-eT,


which leads to the Heikes formula of Eq. (REF ) [1]},
and
the chemical potential in the numerator
is a thermodynamic quantity, which does not give the anisotropic property.
Note that other parameters to produce the anisotropy such as velocity and relaxation time are cancelled out in Eq. (REF ).
Therefore, the thermopower anisotropy,
which should be unity within the Heikes formula,
is an indicator for localized electronic state.
